Superconducting magnet systems having relatively large energies are currently used in many applications. For example, superconducting magnet systems, storing energies of up to 10 M Joules, are constructed for Magnetic Resonance Imaging (MRI) systems which are now being routinely used in large numbers in clinical environments for medical imaging. A part of such an MRI system is a superconducting magnet system for generating a uniform magnetic field.
Superconducting magnets tend to be inherently unstable in that the temperature of a winding region of the magnet can rise relatively rapidly, due to a disturbance within the magnet itself or due to a cause external to the magnet. Such a temperature rise causes a quenching of that winding region, i.e., the superconducting winding goes from its superconducting state of essentially zero resistance to a resistive state. When such region gets hot very rapidly the stored energy within the magnet tends to become dissipated rapidly into that finite resistive region and may severely damage the magnet, even in some cases causing an actual melting of the superconducting wires in the winding.
Accordingly, it is necessary to provide protection for the winding, as well as for the winding of the persistent mode switch used in conjunction with the magnet, in order to ensure safe dissipation of the stored energy in case of such an instability. Furthermore, the magnetic field of the system may have to be discharged for reasons other than a malfunction of the magnet itself. For instance, it may be desirable to discharge the magnetic field if a ferromagnetic object is incidentally drawn into the strong field region.
Accordingly, protection and discharge of the magnet are often achieved by the use of heaters which are located both on the windings themselves and on the associated persistent mode switch (also called superconducting switch). If an instability occurs at one particular winding or winding region of the magnet, all of the heaters used thereon can be controlled into operation so as to quench all the other regions of the magnet, i.e., the stored energy dissipation does not occur only at the particular winding region where an initial quench has occurred but rather is dissipated throughout the entire magnet and thus the damage to any particular winding region can be prevented. However, quenching protection circuits using heaters sometimes are not very stable.
For these and other reasons, there is a need for embodiments of the invention.